As an essential cofactor of glutathione peroxidase, selenium is an important antioxidant. Selenium is incorporated into proteins to make selenoproteins, which are important antioxidant enzymes. The antioxidant properties of selenoproteins help prevent cellular damage from free radicals. Other selenoproteins help regulate thyroid function and play a role in the immune system, DNA repair, and the detoxification of heavy metals. High doses of vitamin C (over one gram) may reduce the absorption of selenium. This mineral is best taken one hour before or 20 minutes after taking vitamin C supplements.

Se-methylselenocysteine (SeMC) is a naturally occurring seleno-amino acid that is synthesized by plants such as garlic and broccoli. SeMC is an organoselenium compound and also more bioabsorbable than other forms. Unlike selenomethionine, which is incorporated into proteins in place of methionine, SeMC is not incorporated into any proteins, thereby being fully available for the synthesis of selenium containing enzymes such as glutathione peroxidase.